The present invention relates generally to wind turbine rotor blades and in particular to wind turbine rotor blades having an acoustic lining. In particular, the present invention relates to an acoustic lining of a wind turbine rotor blade for reducing the noise emission from the wind turbine.
Noise emission from technical installations, particularly power systems, is a safety and environmental concern. Legislation dictates the admissible sound pressure level that a noise source in a certain location may continuously make so that reduced noise emission is a key requirement for low impact environmental integration. Regulations governing these levels vary currently from country to country. Typically, the maximum allowable noise values depend on the character of the surroundings and the time of day. For example, 55 dB(A) may be allowed in a prevailing industrial surrounding during the day, whereas only 35 dB(A) may be allowed in exclusively residential surroundings during the night. These regulations define environmental requirements and acoustical targets for wind turbine installations.
In view of the above, noise generated by wind turbines is an important issue in wind turbine design. The noise generated by the wind turbine is partly mechanical and partly aerodynamic. Mechanical noise is generated mainly from machinery in the nacelle, particularly the gearbox and the generator, although there may also be contributions from cooling fans, auxiliary equipment (such as pumps and compressors), bearings and the yaw system. A major cause for aerodynamic noise of a wind turbine is the operation of the rotor blades. In particular, several mechanisms for generating aerodynamic noise at the blade are known: blade airfoil thickness and in-flow turbulence noise, laminar layer oscillation noise, blade flow separation oscillation noise, and turbulent boundary layer edge interaction noise.